In continuous coking or charring processes, one of the most difficult operations is to pass air upwardly or to downdraft gas through a moving fuel bed when the fuel bed contains an excess of minus 1/4 inch size fraction coal. When attempt is made to pass air upwardly through such a bed the usual results are reduced production rates, excessive fluidization, lower coal to coke or char yields, lower fixed carbon recovery and excessive fly ash. Downdrafting gas through a bed with excessive fines generally causes an excessive quantity of fines to be drawn into the under grate zones where a fire hazard exists if the sifting removal capacity is exceeded.
In order to overcome the coal size limitations imposed upon prior art processes, we have devised a method whereby more than 25% coal fines can be utilized by selective column or row loading of coarse and fine sizes. This method for producing chemical grade coke or char is especially applicable where the char size reduction is required either for direct use or for char briquetting or where a binder is used to produce metallurgical coke, such as in Mansfield et al U.S. Pat. No 3,969,088 (supra). This improved method not only allows the use of coals containing fines (minus 1/4 inch by 0) in the range of 40 to 60%, but it also improves the chemical quality of the coke thus produced.
Among the advantages of this apparatus and method are the ability to use more economical coals for coke production without reducing production rate or increasing fluidization which produces particulate emissions. Modern continuous mining methods oftentimes produce up to 60% coal fines, and consequently a process such as this which can use all of the normally produced fines in the coal tends to be more economical than a process requiring double screen coal size, which limits the amount of 1/4 by 0 to approximately 25%. The market price for double screen sizes of coal is normally greatly higher than the as mined coal sizes. Other advantages of the process are greater yield of coke per ton of coal fed and greater density of the coke product. A further feature of the invention is the use of lower coal to air ratio than in the most nearly comparable prior art processes, which results in a greater recovery of fixed carbon. This is due to characteristics of the coal in the coarse columns or rows which cokes with less air than required when greater than 25% fines are intimately mixed or layer loaded, as in the prior art. In addition, this improved method reduces the amount of crushing required to produce small size coke when needed either for direct use of for briquetting with a binder.